An amplifier is a device that enables an input signal to control power from a source independent of the signal and thus is capable of delivering an output that bears some relationship to, and is generally greater than, the input signal. For power amplifiers the main considerations are output power and efficiency. Most modern power amplifiers use three-terminal solid state devices although vacuum technology is still used. These three terminal devices (e.g. bipolar junction transistors (BJTs), vertical and lateral MOSFETs, IGBTs, GaN HEMTs etc.) are typically used in configurations where one terminal is common between the low power input and high power output, one terminal connects only to the low power input and one terminal connects only to the high power output. For example, for a BJT used in a common emitter configuration, the emitter terminal is the common terminal, whereas the base is connected only to the low power input and the collector connects only to the high power output. For a MOSFET connected in a common source configuration the source is the common terminal, the gate connects only to the low power input and the drain connects only to the high power output. In many cases two physical connections are made to the common terminal to avoid common inductance between the input and output (e.g., Kelvin connection); however, this does not change the characterization of the device as a three terminal device. The output power capability and efficiency of power amplifiers employing three terminal devices are impacted significantly by the ability to provide a well-controlled signal between the input terminal and the common terminal.